1. Technical Field
The present invention relates to an optical disk apparatus, and more particularly a technique for demodulating address information from an MSK-modulated wobble signal.
2. Related Art
A scheme for embedding address information about an optical disk into a wobble signal of the optical disk has hither to been adopted. An MSK (Minimum Shift Keying) modulation scheme, a PSK (Phase-Shift Keying) modulation scheme, or an FSK (Frequency-Shift Keying) modulation scheme is used alone as the scheme. In addition to these schemes, there is also a system which uses both the MSK modulation scheme, as in the case of a Blu-ray disk, and an HMW (Harmonic Wave) modulation scheme. In the MSK modulation scheme, frequency modulation is effected while one frequency is taken as being identical with that of a reference carrier signal and another frequency is taken as being 1.5 times the frequency of the reference carrier signal. The reference carrier signal is assumed to have a signal waveform of cos(ωt), code data “0” assumes a signal waveform of cos(ωt) or an inverted signal waveform of −cos(ωt), and code data “1” assumes a signal waveform of cos(1.5ωt) or an inverted signal waveform of −cos(1.5ωt). The MSK modulation mark is formed from a three-carrier period section having a signal waveform of cos(1.5ωt), −cos(ωt), and −cos(1.5ωt).
Japanese Patent Laid-Open Publication No. 2004-310958 describes, at the time of demodulation of address information from an MSK-modulated wobble signal, multiplying the wobble signal by a carrier signal (a reference clock signal) generated from the wobble signal by a PLL circuit, to thus detect, as an MSK modulation mark, a point where a value determined by adding up multiplication results at every carrier period becomes negative. The Patent Publication also provides a description of inputting a multiplication result to a low-pass filter, to thus detect, as an MSK modulation mark, a point where a value output from the low-pass filter becomes negative.
As mentioned above, the MSK modulation mark can be detected by the wobble signal and the reference clock signal, and easy, reliable detection of a modulation mark is desired. In association with an increase in the density of an optical disk, the influence of crosstalk from an adjacent track cannot be ignored. Occurrence of shift of the phase of a wobble signal attributable to crosstalk is also assumed. Therefore, reliable detection of an MSK modulation mark without being affected by crosstalk is also required.